Double ion exchange of an ultrafiltrate derived from an electrodeposition bath

ABSTRACT

This invention relates to a method of treating and controlling the composition of an electrodeposition bath. The process of the invention comprises ultrafiltration in combination with double ion exchange of the filtrate derived from the ultrafiltration process.

United States Patent 1151 3,663,403

Christensen et a]. 1 May 16, 1972 [54] DOUBLE ION EXCHANGE OF AN [56] References Cited ULTRAFILTRATE DERIVED FROM AN UNITED STATES PATENTS ELECTRODEPOSITION BATH 3,355,373 1 1/1967 Brewer et al ..204/l8l [72] inventors: Roger M. Chrlstenson, Gibsonia', Robert 3,419,488 12/1968 Cooke ..204/l8l R- Z Kensington. both of 3,556,970 1 1971 Wallace et al. 204/18! [73] Assignee: PPG Industries, Inc., Pittsburgh, Pa. FOREIGN PATENTS R APPLICATIONS 7, 97 [22] Wed Nov 2 l 0 1,071,458 6/1967 Great Britain ..204/1s1 [2|] Appl.No.: 93,088

Relaed us. Application Data Primary Exann'nen-Howard S. Williams Attorney-Chisholm and Spencer [63] Continuation-impart of Ser. No. 71,984, Sept. l4,

I970, which is a continuation-in-part of Ser. No. [57] ABSTRACT 879,769, Nov. 25, I969, abandoned.

This invention relates to a method of treating and controlling [52] US. Cl .1 4/ 204/130 04/3 0 the composition of an electrodeposition bath. The process of H 5/02 C23) 13/00 the invention comprises ultrafiltration in combination with [58] 0' Search "204 8] double ion exchange of the filtrate derived from the ultrafihra tion process.

Claims, 3 Drawing Figures 1 ea c cmwoe EXCHANGE Jl I A L -I E H l 1 5 5 ccmc TE [011 rim? 5 F9 llllllllllllllln'rnuxl 3'1: l [ULTRAFILTIBATE] 4 l \'!J 5 l 5 Patented May 16, 1972 .2 Sheets-Sheet l4 FIG-I FIG. 2

INVENTORS 0053 M- chasm/so KQIZHBT 1e. Zu/ncK DOUBLE ION EXCHANGE OF AN ULTRAFILTRATE DERIVED FROM AN ELEC'I'RODEPOSITION BATH CROSS-REFERENCE TO RELATED APPLICATIONS STATE OF THE ART Electrodeposition has become a widely commercially accepted industrial coating technique. The coatings achieved have excellent properties for many applications and electrodeposition results in a coating which does not run or wash off during baking. Virtually any conductive substrate may be coated by electrodeposition, the most commonly employed substrates being metals.

In the electrodeposition process, the articles to be electrocoated are immersed in an aqueous dispersion of solubilized, ionized, film-forming materials such as synthetic organic vehicle resins. An electric current is passed between the article to be coated, serving as an electrode, and a counter electrode to cause deposition of a coating of the vehicle resin on the article. The article is then withdrawn from the bath, usually rinsed and then the coating either air-dried or baked in the manner of a conventional finish.

A major problem in the continuous electrodeposition process has been the control of the electrodeposition bath to maintain initial bath properties. The solubilized electrodepositable vehicle resin may be characterized as a polyelectrolyte, that is, a polyacid or a polybase solubilized by a water-soluble base in the first instance, and a water-soluble acid in the second instance. When the vehicle resin is coated upon an article serving as an anode in the case of the polyacid, and the cathode in the case of a polybase, there remains in the solution a counter-ion, which is the base or acid used to solubilize the resin. The control or removal of excess counter-ion has been attacked by many means. These include circulating the bath through an ion-exchange resin, using a counter-ion deficient feed stock which will scavenge excess counter-ions, circulating the bath through a dialysis unit, concentrating the counter-ion in an electrodialysis cell formed by surrounding the electrode with a semi-permeable membrane, and the use of a vapor-liquid separation process.

While these counter-ion control means have permitted continuous bath operations, it has been seen that it remains extremely difficult to control operating tanks to maintain their initial paint properties. Almost without exception, the tank never has better properties than the day it is filled and from that time on, efforts must be directed toward minimizing loss of coating properties. It has been found that as tanks operate, chemicals from various sources tend to accumulate in the electrodeposition tank. The most probable sources of the contamination are the chemicals on the object to be painted, water added to the tank, or chemicals absorbed from the air or from the paint itself. Regardless of the technique employed to remove counter-ions and some other objectionable ions, the deterioration of film properties has been frequently encountered.

ln copending application, Ser. No. 814,789, filed Apr. 9, 1969, there is disclosed a method of controlling the composition of electrodeposition baths by an ultrafiltration process. In the ultrafiltration process, excellent control of the bath composition and removal of objectionable accumulated materials has been achieved by a selective filtration process, that is, a process which selectively removes low molecular weight materials from the bath composition. This selective filtration process removes excess counter-ions and thus serves as a method of conventional bath control; but, in addition, this method removes other excess materials and/or contaminants from the bath, thus permitting fewer hannful bath constituents than had heretofore been possible.

ln copending application, Ser. No. 879,769, filed Nov. 25, 1969, now abandoned there is disclosed that further economies in the electrodeposition process, as well as greater flexibility in the operation of the electrodeposition process, are achieved by passing the effluent filtrate from the ultrafiltration process through an ion exchange media, thereby removing at least a portion of objectionable ion from the ultrafiltrate.

DESCRIPTION OF THE INVENTION It has now been found that by passing the effluent filtrate from the ultrafiltration process (this filtrate hereinafter being referred to as ultrafiltrate) through a multiplicity of ion exchange media, that is, both cationic and anionic ion exchange media, substantially all or at least a substantial portion of the objectionable ions contained in the ultrafiltrate can be removed.

The process of this invention allows for greater flexiblity in the operation of the electrodeposition process. This process is useful in removing excess counter-ions and carbonate ions, as well as deleterious ions introduced from external sources, such as metal pretreatments or ions entering the bath in the initial charge or in the addition of makeup materials, which if allowed to accumulate frequently tend to reduce the quality of the film deposited.

The process of the invention has advantages over either ultrafiltration or ion exchange alone, including greater flexibility in bath control.

Specifically, as compared to ion exchange alone, ion removal is accomplished without the ion-exchange media coming in contact with the vehicle resin. This is an important factor since the efficacy of the ion exchange media in controlling the electrodeposition process has always been severely hampered by the presence of the vehicle resin which is ionic in character as well as pigment and other components which coat or deposit upon or clog the ion-exchange media in time. Most vehicle resins, in fact, will precipitate on ion exchange media of the opposite charge and will reduce its efficacy in a relatively short time.

The process of the invention has substantial advantages over ultrafiltration alone. For example, while it is desirable to operate an electrodeposition ultrafiltration system as a closed loop system, utilizing ultrafiltrate to rinse dragout back into the electrodeposition bath, if the cycle is totally closed ion species and especially counter-ions soon build to levels which affect coating performance. Therefore, practically, purported closed cycle systems have been required to divert substantial amounts of ultrafiltrate for bath control, thereby losing valuable chemical species contained in the ultrafiltrate. The use of ion exchange in this system allows for the removal of the objectionable ionic species prior to returning ultrafiltrate to the system, thus allowing a more substantially closed loop trou blefree system. Further economies are realized by the system of the invention since the effluent of the ion exchange system, when returned to the electrodeposition bath recycles desirable non-ionic low molecular weight modifiers which may initially be removed by ultrafiltration, such as organic solvents and low molecular weight organic resins such as some melamine resins. This recycling now is achieved without returning undesirable ionic species.

While it is presently preferred to use the ion-exchanged ultrafiltrate as rinse water for the reasons set forth above, the ion-exchanged ultrafiltrate may be utilized in other manners, such as being directly returned to the electrodeposition bath, which still provides economies in material use; as rinse water which is withdrawn from the system; or it may from time to time be treated as waste when it is advantageous to utilize this mode for bath control not otherwise obtainable, such as solids control or variation or the removal of contaminants, including low molecular weight non-ionic species.

The process of the invention provides means for controlling and reducing environmental pollution, especially water pollution, since the final resultant waste material has been depleted of a substantial amount of chemical constituents which otherwise would enter a waste disposal system.

The ultrafiltrate can be ion exchanged by contacting both anion and cation exchange resins either in a mixed bed or separately in order that the progress of ion removal may be monitored, if desired, by measuring pH, conductivity, or quantitative or qualitative tests for specific ions, as well as other ion sensing techniques known in the art. if desired, automatic control may be accomplished by feedback of this data.

For example, where the solubilized electrodepositable resin is a polyacid solubilized with a base, the pH can be reduced and cations can be removed from the aqueous ultrafiltrate by contacting the ultrafiltrate with a cation exchange resin in the hydrogen form. The time of contact is controlled so as to maintain the pH of the ultrafiltrate at the desired level, generally within a range of 2 to 5. The resultant filtrate is then subjected to contact with an anion exchange resin to remove anions.

The desired pH level, the rate at which the ultrafiltrate is treated and the actual time of contact to be maintained between the ultrafiltrate and the ion-exchange resins all vary with several factors, including the type of ion exchange resin employed and that nature the the cations and anions, and the desired end use of the resultant filtrates.

Any type of cation exchange resin can be employed as long as the composition contacted can be made to acquire the desired pH or conductivity within any reasonable time. The scope of cation exchange resins which may be employed is substantially larger than that which can be employed in directly contacting the electrodeposition bath composition since the stability of the coating composition in contact with the ion exchange resin is of no moment. It is preferred to use a cation exchange resin having carboxylic acid groups as func' tional sites. Examples of the weak carboxylic acid type resins commercially available which are preferred are Amberlyte [RC-50" and lonac C-270". Strong ion exchange resins such as sulfonic acid type cation exchange resins may also be employed where relatively short contact times are desired. The use of strong exchange resins in the past has been difficult since, in contact with the resin of the bath, stability problems were common.

The anion exchange medium which may be employed is virtually any anion exchange resin which, when contacted with the ultrafiltrate, causes the ultrafiltrate to acquire the desired pH or conductivity within any reasonable time. The presently preferred anion exchange resins are weak polyamine type resins such as phenolic polyamine resins. Commercially available polyamine resins include Duolite A-7, Amberlite lR-4B, and "Amberlite lR-45." Strong anion exchange resins include quaternary ammonium group-containing resins such as Amberlite IRA-4 l0," after conversion to the hydroxyl form. Again, the choice of ion exchange resin is not governed by bath resin sensitivity since the exchange resin does not contact the bath composition.

The treatment may be carried out either on a batch basis or a continuous basis in concurrent or intermittent operation with the ultrafiltration process utilized in controlling the electrodeposition bath. In the batch operation, an ultrafiltrate composition which has a variant pH or an excess of cations and/or anions is easily treated by mixing and stirring the exchange resins, either in admixture or in sequence, with the ultrafiltrate for the desired time and separating the resin from the composition.

in a continuous process, a portion of the ultrafiltrate is contacted with the ion exchange resins and returned to the electrodeposition bath, either directly or after it has been utilized to rinse dragout. Alternatively, the product of the separation can be treated as waste material. The rate of withdrawal may be correlated with the rate at which ions are being formed or introduced in the electrodeposition process and the rate at which additional quantities of coating composition are added to the bath. The time of contact, and thus the pit or conductivity of the resultant composition are regulated by adjusting the rate of input and return and the length of the resin bed through which the ultrafiltrate is passed, or the ratio of ultrafiltrate to ion exchange resin in a batch ion exchange.

in either cue, regeneration of the ion exchange resins is easily accomplished by washing, first with water and then with dilute acids, for example, 5 percent aqueous hydrochloric acid in the case of cation exchange resins and appropriate bases in the case of anion exchange resins.

In a continuous operation, it is desired to utilize two or more columns containing ion exchange resin so a fresh column is immediately available at all times.

in the attached drawing, FIG. 3, an apparatus used to carry out the method of the invention is schematically illustrated. The electrodeposition bath 1 contains an aqueous elec trodepositable composition from which films are deposited, using suitable apparatus (not shown). A portion of the bath may be continuously or intermittently withdrawn through an outlet and valve 2 and passed through a line 3 to an ultrafilter 4. Here, in the ultrafiltration process, water, excess counterions and other low molecular weight species, if present, are separated from the resin, pigment, and other higher molecular weight components which may be present in the bath composition. The concentrate or retentate may be returned to the bath through line 26 and valve 28. The ultrafiltrate is removed from the ultrafilter, passing through line 5. Through the use of a valve 6, the ultrafiltrate may be directed either unidirectionally or proportionally in either an intermittent or a continuous fashion to drain 7, for use as rinse material through line 25 or to the mixed anion-cation exchange media through line 8. The ultrafiltrate is then passed through an ion exchange column containing mixed anion and cation exchange resins 9 by opening valves l0 and H and closing valves 12 and 13. After the exchange capacity of column 9 has been depleted, the bath composition is passed through ion exchange column 14 by closing valves 10 and 11 and opening valves 12 and I3. Column 9 is then disconnected from the system and the resin regenerated. The column is then replaced in the system. By alternating columns in this manner, the exchange treatment may be carried out continuously. The resultant filtrate from the ion exchange columns is passed through line 15 to valve 16, where it is directed either unidirectionally or proportionally to drain 17 or through the lines 18 and 20 to either a rinse station for rinsing dragout in such a manner that the rinse-containing dragout is returned to the bath 23 or for rinsing dragout in a manner 22 so that it does not return directly to the bath.

Alternatively, the effluent and/or ultrafiltrate may be passed either directionally or proportionally directionally back into the bath through line 27 and valve 29 or in conjunction with the concentrate or retentate through valves 28 and 29. As stated, this drawing is schematic and does not purport to show pumping means and apparatus which are known in the art. Likewise, various other alternatives in the flow system are within the skill of the art.

Ultrafiltration encompasses all membrane-moderated, pressure-activated separations wherein solvent or solvent and smaller molecules are separated from modest molecular weight macromolecules and colloids. The tenn "ultrafiltration is generally broadly limited to describing separations involving solutes of molecular dimensions greater than about ten solvent molecular diameters and below the limit of resolution of the optical microscope, that is, about 0.5 micron. In the present process, water is considered the solvent.

The principles of ultrafiltration and filters are discussed in a chapter entitled Ultrafi1tration" in the Spring, 1968, volume of Advances In Separations And Purifications, E. S. Perry, Editor, John Wiley & Sons, New York, as well as in Chemical Engineering Progress, Vol. 64, Dec., 1968, pages 3| through 43, which are hereby incorporated by reference.

The basic ultrafiltration process is relatively simple. Solution to be ultrafiltered is confined under pressure, utilizing, for example, either a compressed gas or liquid pump in a cell, in contact with an appropriate filtration membrane supported on a porous support. Any membrane or filter having chemical integrity to the system being separated and having the desired separation characteristic may be employed. Preferably, the contents of the cell should be subjected to at least moderate agitation to avoid accumulation of the retained solute on the membrane surface with the attendant binding of the membrane. Ultrafiltrate is continually produced and collected until the retained solute concentration in the cell solution reaches the desired level, or the desired amount of solvent plus dissolved low molecular weight solute is removed. A suitable apparatus for conducting ultrafiltration is described in U. S. Pat. No. 3,495,465, which is hereby incorporated by reference.

There are two types of ultrafiltration membrane. One is the microporous ultrafilter, which is a filter in the traditional sense, that is, a rigid, highly voided structure containing interconnected random pores of extremely small average size. Through such a structure, solvent (in the case of electrodeposition, water) flows essentially viscously under a hydraulic pressure gradient, the flow rate proportional to the pressure difference, while dissolved solutes, to the extent that their hydrated molecule dimensions are smaller than the smallest pores within the structure, will pass through, little impeded by the matrix. Larger size molecules, on the other hand, will become trapped therein or upon the external surface of the membrane and will thereby be retained. Since the microporous ultrafilters are inherently susceptible to internal plugging or fouling by solute molecules whose dimensions lie within the pore size distribution of the filter, it is preferred to employ for a specific solute a microporous ultrafilter whose mean pore size is significantly smaller than the dimensions of the solute particle being retained.

In contrast, the diffusive ultrafilter is a gel membrane through which both solvent and solutes are transported by molecular diffusion under the action of a concentration or activity gradient. In such a structure, solute and solvent migration occurs via random thermal movements of molecules within and between the chain segments comprising the polymer network. Membranes prepared from highly hydrophilic polymers which swell to eliminate standard water are the most useful diffusive aqueous ultrafiltration membranes. Since a diffusive ultrafilter contains no pores in the conventional sense and since concentration within the membrane of any solute retained by the membrane is low and time-independent, such a filter is not plugged by retained solute, that is, there is no decline in solvent permeability with time at a constant pressure. This property is particularly important for a continuous concentration or separation operation. Both types of filters are known in the art.

The presently preferred ultrafilter is an anisotropic membrane structure such as illustrated in FIG. I. This structure consists of an extremely thin, about one-tenth to about to micron layer, of a homogeneous polymer 1 supported upon a thicker layer of a microporous open-celled sponge 2, that is, a layer of about 20 microns to about 1 millimeter, although this dimension is not critical. If desired, this membrane can be further supported by a fibrous sheet, for example, paper, to provide greater strength and durability. These membranes are used with a thin film or skin side exposed to the high pressure solution. The support provided to the skin by the spongy substrate is adequate to prevent film rupture.

Membranes useful in the process are items of commerce and can be obtained by several methods. One general method is described in Belgian Pat. No. 721,058. This patent describes a process which, in summary, comprises (a) forming a casting dope of the polymer in an organic solvent, (b) forming a film of the casting dope, and (c) preferentially contacting one side of said film with a diluent having high compatibility with the casting dope to effect precipitation of the polymer immediately upon coating the cast film with the diluent.

The choice of a specific chemical composition for the membrane is determined to a large extent by its resistance to the chemical environment. Membranes can be typically prepared from thermoplastic polymers such as polyvinyl chloride, polyacrylonitrile, polysulfones, poly( methyl methacrylate),

polycarbonates, poly(n-butyl methacrylate), as well as a large group of polymers formed from any of the monomeric units of the above polymers, including "Polymer 360, a polysulfone copolymer. Cellulosic materials such as cellulose acetate may also be employed as membrane polymers.

Some examples of specific anisotropic membranes operable in the process of the invention include Diaflow membrane ultrafilter PM-30, the membrane chemical composition of which is a polysulfone copolymer, Polymer 360, and which has the following permeability characteristics:

Solute Retention Characteristics Flux Molecular Percent (gal./sq.ft./day at Solute Weight Retention 30 psi, 1.0% solute) Cytochrome C 12,600 50 100 aChymotr-ipsinagen 24,000 22 Ovalburnin 45,000 45 This membrane is hereinafter referred to as Membrane B.

Dorr-Oliver BPA type membrane, the membrane chemical composition of which is phenoxy resin (polyhydroxy ether), and which has the following permeability characteristics:

The microporous ultrafilters are generally isotropic structures, thus flow and retention properties are independent of flow direction. it is preferred to use an ultrafilter which is anisotropic in its microporous membrane structure, FIG. 2. In such a membrane, the pore size increases rapidly from one face to the other. When the fine-textured side 4 is used in contact with the feed solution, this filter is less susceptible to plugging since a particle which penetrates the topmost layer cannot become trapped in the membrane because of the larger pore size 5 in the substrate.

The process of the invention may be operated as either a batch or a continuous process. In batch selective filtration or batch ultrafiltration a finite amount of material is placed in a cell which is pressurized. A solvent and lower molecular weight solutes are passed through the membrane. Agitation is provided by a stirrer, for example, a magnetic stirrer. Obviously, this system is best used for small batches of material. In a process requiring continuous separation, a continuous selective filtration process is preferred. Using this technique, material is continuously recirculated under pressure against a membrane or series of membranes through interconnecting flow channels, for example, spiral flow channels.

Likewise, the ultrafiltration process may be conducted as either a concentration process or a diafiltration process. Concentration involves removing solvent and low molecular weight solute from an increasingly concentrated retentate. Filtration flow rate will decrease as the viscosity of the concentrate increases. Diafiltration, on the other hand, is a constant volume process whereby the starting material is connected to a reservoir of pure solvent, both of which are placed under pressure simultaneously. Once filtration begins, the pressure source is shut off in the filtration cell and thus, as the filtrate is removed, an equal volume of new solvent is introduced into the filtration cell to maintain the pressure balance. The configuration of the filter may also vary widely and is not limiting to the operation of the process. The filter or membrane may, for example, be in the form of a sheet, tubes or hollow fiber bundles, among other configurations.

Under ideal conditions, selected low molecular weight solutes would be filtered as readily as solvent and their concentration in the filtrate is equal to that in the retentate. Thus, for example, if a material is concentrated to equal volumes of filtrate and retentate, the concentration of low molecular weight solute in each would be the same.

Using diafiltration, retentate solute concentration is not constant and the mathematical relationship is as follows:

lo l) I B where C is the initial solute concentration, C, is the final solute concentration of the retentate, V is the volume of solute delivered to the cell (or the volume of the filtrate collected), and V, is the initial solution volume (which remains constant).

Electrodepositable compositions, while referred to as solubilized, in fact are considered a complex solution, dispersion or suspension or combination of one or more of these classes in water, which acts as an electrolyte under the influence of an electric current. While, no doubt, in some circumstances the vehicle resin is in solution, it is clear that in some instances and perhaps in most the vehicle resin is a dispersion which may be called a molecular dispersion of molecular size between a colloidal suspension and a true solution.

The typical industrial electrodepositable composition also contains pigments, crosslinking resins and other adjuvants which are frequently combined with the vehicle resin in a chemical and a physical relationship. For example, the pigments are usually ground in a resin medium and are thus wetted with the vehicle resin. As can be readily appreciated then, an electrodepositable composition is complex in terms of the freedom or availability with respect to removal of a component or in terms of the apparent molecular size of a given vehicle component.

As applied to the process of this invention, ultrafiltration comprises subjecting an electrodepositable composition, especially after it has been employed in a coating process or aged, which inherently causes contaminants and other low molecular weight materials to accumulate in the bath, such as metal pretreatment chemicals, water, absorbed C0, (either dissolved or, more likely, combined as an aminic salt or carbonate), neutralizing agent, organic solvent and ions such as formate, chromate, phosphate, chloride and sulfate, for example, to an ultrafiltration process employing an ultrafilter, preferably a diffusive membrane ultrafilter selected to retain the solubilized vehicle resin while passing water and low molecular weight solute, especially those with a molecular weight below about 500. As previously indicated, the filters discriminate as to molecular size rather than actual molecular weight, thus, these molecule weights merely establish an order of magnitude rather than a distinct molecular weight cut-off. Likewise, as previously indicated, the retained solutes may, in fact, be colloidal dispersions or molecular dispersions rather than true solutes.

In practice, a portion of the electrodepositable composition may be continuously or intermittently removed from the electrodeposition bath and passed under pressure created by a pressurized gas or by means of pressure applied to the contained fluid in contact with the ultrafilter. Obviously, if desired, the egress side of the filter may be maintained at a reduced pressure to create the pressure difference.

The pressures necessary are not severe. The maximum pressure, in part, depends on the strength of the filter. The minimum pressure is that pressure required to force water and low molecular weight solute through the filter at a measurable rate. With the presently preferred membranes, the operating pressures are between about 10 and psi, preferably between about 25 and 75 psi Under most circumstances, the ultrafilter should have an initial flux rate, measured with the composition to be treated of at least about 3 gal./sq.ft./day (24 hours) and preferably at least about 4.5 gal./sq.ft./day.

As previously indicated, the bath composition should be in motion at the face of the filter to prevent the retained solute from impeding the flow through the filter. This may be accomplished by mechanized stirring or by fluid flow with a force vector to the filter surface.

The retained solutes comprising the vehicle resin are then returned to the electrodeposition bath. If desired, the concentrate may be reconstituted by the addition of water either before entry to the bath or by adding water directly to the bath.

If there is present in the bath desirable materials which, because of their molecular size, are removed in the ultrafiltration process, these may likewise be returned to the bath either directly to the retained solute before entry to the bath, in the makeup feed as required, or independently.

A number of electrodepositable resins are known and can be employed to provide the electrodepositable compositions which may be utilized within the scope of this invention. Virtually any water-soluble, water-dispersible or water-emulsifiable polyacid or polybasic resinous material can be electrodeposited and, if film-forming, provides coatings which may be suitable for certain purposes. Any such electrodepositable composition is included among those which can be employed in the present invention, even though the coating obtained might not be entirely satisfactory for certain specialized uses.

Presently, the most widely used electrodeposition vehicle resins are synthetic polycarboxylic acid resinous materials. Numerous such resins are described in US. Pat. Nos. 3,441,489; 3,422,044; 3,403,088; 3,369,983 and 3,366,563, which are incorporated by reference. These include a reaction product or adduct of a drying or semi-drying oil fatty acid ester with a dicarboxylic acid or anhydride. By drying oil or semi-drying oil fatty acid esters are meant esters of fatty acids which are or can be derived from drying oils or semi-drying oils, or from such sources as tall oil. Such fatty acids are characterized by containing at least a portion of polyunsaturated fatty acids. Preferably, the drying oil or semi-drying oil per se is employed.

Also included among such esters are those in which the esters themselves are modified with other acids, including saturated, unsaturated or aromatic acids or an anhydride thereof. The acid-modified esters are made by transesterification of the ester, as by forming a dior monoglyceride by alcoholysis, followed by esterification with the acid; they may also be obtained by reacting oil acids with a polyol and reacting the acid with the partial ester. In addition to glycerol, alcoholysis can be carried out using the other polyols such as trimethylolpropane, pentaerythritol, sorbitol and the like. If desired, the esters can also be modified with monomers such as cyclopentadiene or styrene and the modified esters produced thereby can be utilized herein. Similarly, other esters of unsaturated fatty acids, for example, those prepared by the esterification of tall oil fatty acids with polyols, are also useful.

Also included within the terms "drying oil fatty acid esters" as set forth herein are alkyd resins prepared utilizing semi-drying or drying oils; esters of epoxides with such fatty acids, including esters of diglycidyl ethers of polyhydric compounds as well as other mono-, diand polyepoxides, semi-drying or drying oil fatty acid esters of polyols, such as butanediol, trimethylolethane, trimethylolpropane, trimethylolhexane, pentaerythritol, and the like; and semi-drying or drying fatty acid esters of resinous polyols such as homopolymers or copolymers of unsaturated aliphatic alcohols, e.g., allyl alcohol or methallyl alcohol, including copolymers of such alcohols with styrene or other ethylenically unsaturated monomers or with non-oil modified alkyd resins containing free hydroxy] groups.

Any alpha, beta-ethylenically unsaturated dicarboxylic acid or anhydride can be employed to produce the reaction products described herein. These include such anhydrides as maleic anhydride, itaconic anhydride, and other similar anhydrides. Instead of the anhydride, there may also be used ethylenically unsaturated dicarboxylic acids which form anhydrides, for example, maleic acid or itaconic acid. These acids appear to function by first forming the anhydride. Fumaric acid, which does not form an anhydride, may also be utilized, although in many instances it requires more stringent conditions than the unsaturated dicarboxylic acid anhydrides or acids which form such anhydrides. Mixtures of any of the above acids or anhydrides may also be utilized. Generally speaking, the anhydride or acid employed contains from four to l2 carbon atoms, although longer chain compounds can be used if so desired.

While the reaction products can be comprised solely of adducts of the fatty acid ester and the dicarboxylic acid or anhydride, in many instances it is desirable to incorporate into the reaction product another ethylenically unsaturated monomer. The use of such monomer often produces films and coatings which are harder and more resistant to abrasion and which may have other similar desirable characteristics.

As shown in the art, it is preferred that in certain instances the neutralization reaction be carried out in such a manner that amido groups are attached to part of the carbonyl carbon atoms derived from the dicarboxylic acid or anhydride.

Compositions within this general class are described in U.S. Pat. Nos. 3,366,563 and 3,369,983.

Another vehicle comprises the fatty acid ester, unsaturated acid or anhydride reaction products and any additional unsaturated modifying materials (as described above), which are further reacted with the polyol.

Essentially any polyol can be employed, but diols are preferred. When higher polyols, such as trimethylolpropane, glycerol, pentaerythritol and the like are utilized, they are employed in small amounts, or in conjunction with the diol, or in the presence of a monohydric alcohol, and are used with adducts having a relatively low proportion of acidic component. Water-insoluble diols are often preferable, and especially desirable water-dispersed compositions for electrodeposition are obtained using 2,2-bis(4-hydroxycyclohexyl)propane (which has given the best results), neopentyl glycol, l ,l 'isopropyl-idene-bis(p-phenyleneoxy)di-2-propanol, and similar diols.

The proportions of the polyol and ester-anhydride adduct which are employed depend upon various factors, but are in general limited only by the need to avoid gelation of the product. The total functionality of the reactants is a guide to detemiining the optimum proportions to be employed, and in most instances should not be greater than about 2.

in many instances, only part of the anhydride groups of the adduct, e.g., about 10 percent, are reacted with the polyol. Of those anhydride groups reacted, it is preferred that only one of the carboxyl groups is esterified in each instance.

The product contains a substantial part of the original acidity derived from the dicarboxylic acid or anhydride; ordinarily the product should have an acid number of at least about 20. To provide a water-dispersed product, such as is used in electrodeposition processes, at least part of the remaining acidic groups are neutralized by reaction of the partially esterified product with a base.

The polyol reaction products and reaction conditions are more fully described in application, Ser. No. 450,205, filed Apr. 22, 1965, now U.S. Pat. No. 3,565,78! as well as the art cited above.

Another type of electrodepositable coating composition which gives desirable results are the water-dispersible coating compositions comprising at least partially neutralized interpolymers of unsaturated carboxylic acids and at least one other ethylenically unsaturated monomer. Preferably, the polymer contains a hydroxyalkyl ester of an unsaturated carboxylic acid. These are employed in the composition along with an amine-aldehyde condensation product, with the interpolymer usually making from about 50 percent to about percent by weight of the resinous composition.

The acid monomer of the interpolymer is usually acrylic acid or methacrylic acid, but other ethylenically unsaturated monocarboxylic and dicarboxylic acids of up to about 6 carbon atoms can also be employed. The hydroxyalkyl ester is usually hydroxyethyl or hydroxypropyl acrylate or methacrylate, but also desirable are the various hydroxyalkyl esters of the above acids having, for example, up to about five carbon atoms in the hydroxyalkyl radical. Monoor diesters of the dicarboxylic acids mentioned are included. Ordinarily, the acid and ester each comprise between about one percent and about 20 percent by weight of the interpolymer, with the remainder being made up of one or more other copolyrnerizable ethylenically unsaturated monomers. The most often used are the alkyl acrylates, such as ethyl acrylate; the alkyl methacrylates, such as methyl methacrylate', and the vinyl aromatic hydrocarbons, such as styrene, but others can be utilized.

The above interpolymer is at least partially neutralized by reaction with a base as described above; at least about 10 percent, and preferably 50 percent or more of the acidic groups are neutralized, and this can be carried out either before or after the incorporation of the interpolymer in the coating composition.

The amine-aldehyde condensation products included in these compositions are, for example, condensation products of melamine, benzoguanamine, or urea with formaldehyde, although other amine-containing amines and amides, including triazines, diazines, triazoles, guanadines, guanamines and alkyl and aryl-substituted derivatives of such compounds can be employed, as can other aldehydes, such as acetaldehyde. The alkylol groups of the products can be etherified by reaction with an alcohol, and the products utilized can be watersoluble or organic solvent soluble.

Electrodeposition compositions comprising the above interpolymers and an amine-aldehyde resin are more fully described in U.S. Pat. No. 3,403,088.

Still another electrodepositable composition of desirable properties comprises an alkyd-amine vehicle, that is, a vehicle containing an alkyd resin and an amine-aldehyde resin. A number of these are known in the art and may be employed. Preferred are water-dispersible alkyds such as those in which a conventional alkyd (such as a glyceryl phthalate resin), which may be modified with drying oil fatty acids, is made with a high acid number (e.g., 50 to 70) and solubilized with ammonia or an amine, or those in which a surface active agent,

such as a polyalkylene glycol (e.g., "Carbowax) is incorporated. High acid number alkyds are also made by employing a tricarboxylic acid, such as trimellitic acid or anhydride, along with a polyol in making the alkyd.

The above alkyds are combined with an amine-aldehyde resin, such as those described hereinabove. Preferred are water-soluble condensation products of melamine or a similar triazine with formaldehyde with subsequent reaction with an alltanol. An example of such a product is hexakis(methoxymethyl)melamine.

The alkyd-amine compositions are dispersed in water and ordinarily contain from about to about 50 percent by weight of amine resin based on the total resinous components.

Yet another electrodepositable composition of desirable properties comprises mixed esters of a resinous polyol. These resin esters comprise mixed esters of an unsaturated fatty acid adduct. Generally the polyols which are utilized with these resins are essentially any polyol having a molecular weight between about $00 and about 5,000. Such resinous polyols include those resinous materials containing oxirane rings which can be opened in, prior to, or during the esterification reaction to provide an apparent hydroxy site. The vehicle resins are formed by reacting a portion of the hydroxyl groups of the polyol with the fatty acid, the ratio of the reactions being such that at least an average of one hydroxyl group per molecule of the polyol remains unreacted. The remaining functionality is then reacted with the unsaturated fatty acid adduct of an olefinically unsaturated dicarboxylic anhydride, such as maleic anhydride, this second esterification reaction being conducted under conditions so that esterification occurs through the anhydride ring, thereby introducing free acid groups into the molecule. Mixed acids of the class described are disclosed in Belgian Pat. No. 641,642, as well as in copending application, Ser. No. 568,144, filed July 27, 1966 now abandoned.

In order to produce an electrodepositable composition, it is necessary to at least partially neutralize the acid groups present with a base in order to disperse the resin in the electrodeposition bath. Inorganic bases such as metal hydroxides, especially potassium hydroxide, can be used. There may likewise be used ammonia or organic bases, especially watersoluble amines, such as, for example, the mono-, diand trilower alkyl amines such as methylamine, ethylamine, propylamine, butylamine, dimethylamine, diethylamine, dipropylamine, dibutylamine, and m-methyl-butylamine, triethylamine, tributylamine, methyldiethylamine, dimethylbutylamine, and the like; cyclic amines such as morpholine, pyrrolidine, piperidine; diamines such as hydrazine, methylhydrazine, 2,3-toluene diamine, ethyl diamine and piperizine and substituted amines such as hydroxylamine, ethanolamine, diethanolamine, butanolamine, hexanolamine and methyldiethanolamine, octanolamine, diglycolamine and other polyglycolamines, triethanolamine, and methylethanolamine, n-amino-ethanolamine and methyldiethanolamine and polyamines such as diethyl triamine.

There may be present in the electrodepositable composition any of the conventional types of pigments employed in the art. There is often incorporated into the pigment composition a dispersing or surface-active agent. Usually the pigment and surface-active agent, if any, are ground together in a portion of the vehicle, or alone, to make a paste and this is blended with the vehicle to produce a coating composition.

In many instances, it is preferred to add to the bath in order to aid dispersibility, viscosity and/or film quality, a non-ionic modifier or solvent. Examples of such materials are aliphatic, naphthenic, and aromatic hydrocarbons or mixtures of the same; monoand dialkyl ethers of glycols, pine oil and other solvents compatible with the resin system. The presently preferred modifier is 4-methoxy-4-methyl pentanone-Z (Penn Oxone There may also be included in the coating composition, if desired, additives such as anti-oxidants. For example, orthoamylphenol or cresol. It is especially advantageous to include such antioxidants in coating compositions which are used in baths which may be exposed to elevated temperatures and with periods of time.

Other additives which may be included in coating compositions, if desired, included, for example, wetting agents such as petroleum sulfonates, sulfated fatty amines, or their amides, esters of sodium isothionates, alkyl phenoxypoly-ethylene alkanols, or phosphate esters including ethoxylated alkylphenol phosphates. Other additives which may be employed include anti-foaming agents, suspending agents, bactericides, and the like.

In formulating the coating composition, ordinary tap water may be employed. However, such water may contain a relatively high level of metals and cations which, while not rendering the process inoperative, may result in variations of properties of the baths when used in electrodeposition. Thus, in common practice, deionized water, i.e., water from which free ions have been removed by the passage through ion exchange resins, is invariably used to make up coating compositions of the instant invention.

In addition to the electrodepositable vehicle resins described above, there may be present in the electrodepositable composition other resinous materials which are non-carboxylic acid materials. For example, as shown above, there may be added up to about 50 percent by weight of an aminealdehyde condensation product.

Other base-solubilized polyacids which may be employed as electrodeposition vehicles include those taught in US. Pat. No. 3,392,165, which is incorporated herein by reference, wherein the acid groups rather than being solely polycarboxylic acid groups contain mineral acid groups such as phosphonic, sulfonic, sulfate and phosphate groups.

The process of the instant invention is equally applicable to cationic type vehicle resins, that is, polybases solubilized by means of an acid, for example, an amine-terminated polyamide or an acrylic polymer solubilized with acetic acid. Another case of such cationic polymers is described in copending application, Ser. No. 772,366, filed Oct. 28, I968 now abandoned.

in a manner similar to the anionic resins described above, the cationic resins may be formulated with adjuvants, such as pigments, solvents, surfactants, crosslinking resins, and the like.

The polyacids are anionic in nature and are dispersed or dissolved in water with alkaline materials such as amines or alkaline metal hydroxides and, when subjected to an electric current, they migrate to the anode. The polybasic resins, solubilized by acids, are cationic in character and when these resins are water-dispersed or solubilized with an acid such as acetic acid, the material deposits on the cathode under an electric current.

The invention is further described in conjunction with the following examples, which are to be considered illustrative rather than limiting, All parts and percentages in the examples and throughout this specification are by weight unless otherwise stated.

atmospheric oxygen at agitation over extended EXAMPLE The electrodepositable composition utilized in this was prepared as follows:

A pigment paste was formulated as follows:

example '20 percent maleic anhydride. percent linseed oil, maleinir/ed oil having a viscosity of l00,000 centipoiles were mixed 20 minutes in a closed container. There was then added:

The above components were ground in a conventional zirco mill to a 7% Hegrnan grind gauge reading.

Paste A was reduced as follows:

Composition B Paste A 395.2 Deionized water 667.4 Potassium hydroxide solution (15 percent in water) 414.8

Composition C Parts by Weight Vehicle resin about at 100 percent solids content 1319.6 Wetting agent (sorbitan monolaurate) 129.6 Hexakis( methoxymethylJmelamine 159.8 Ethyl cellosolve 67.0

were mixed minutes and there was added:

Composition 8 1477.4

The composition was again mixed 10 minutes and there was added:

Deionized water 1015.2

The composition was again mixed 10 minutes and then there was added:

Paste A 555.2

There was 10 more minutes mixing and added:

Deionized water 142.4

Composition C had the following characteristics:

Solids content (percent) Pigment-to-binder ratio Composition C was reduced to produce Composition D (below):

Composition D Parts By Weight Composition C 2930.0 Deionized water 8070.0

Composition D had the following properties:

pH 9.45 Conductivity. pmhos/cm., 75 F. 2300 Total solids, content (percent) 12.2 Ash 1.15 MEQ"/l00 grams total 6.40 MEQ'I grams solids 52.5 Nitrogen content (percent) 0.24 CO, (ppm) Ethyl Cellosolve (percent) 042 Milliequivalents of blue The above composition, after repeated turnovers, resulted in electrodeposited films containing roughness patterns. The composition was further intentionally contaminated with 10 percent excess potassium hydroxide and then subjected to ul trafiltration filtering of 70 percent of the volume, utilizing Membrane XPA, described above, at 50 p.s.i. The ultrafiltrate was then subjected to ion exchange with a 50-50 mixed bed of Amberlite [RC-50(H cation exchange resin, and Amberlite 1R45(OH), anion exchange resin, the ion exchange resins having previously been washed to 10 percent slurry conductivities of 8.6 and 14.4 micromhos/cm. respectively. The ratio of ultrafiltrate to mixed ion exchange resin was approximately 5 to l and the contact time was approximately 30 minutes. Properties of the ultrafiltrate before and after ion exchange were as follows:

Before After pH 8.95 6.40 Conductivity 75 F. (micromhos/cm. 1640 186 MEQ base/liter 23.6 3.6

The concentrate from the ultrafiltration step was reconstituted with the ion exchanged ultrafiltrate. The properties of the paint were as follows:

Melamine loss, based on nitrogen analysis, was 19 percent. Solvent loss was 28 percent. A portion of these losses were probably transfer losses. 1f the bath had been reconstituted with deionized water these losses would have been 40 percent and 50 percent respectively. Thus, the ion exchange operation saved the loss of valuable material and avoided a difficult waste disposal problem.

Panels coated from the reconstituted paint had excellent appearance on zinc phosphated steel, bare steel and aluminum. Under the same coating conditions, panels coated from the original material were rough and patchy, particularly on bare steel and aluminum.

Other electrodepositable compositions, such as those hereinabove described, can be substituted for those exemplified. Likewise, various ultraftlters, ion exchange resins, and method variations may be employed to obtain the improvements hereinabove described.

According to the provisions of the Patent Statutes, there are described above the invention and what are now considered its best embodiments; however, within the scope of the appended claims, it is to be understood that the invention can be practiced otherwise than as specifically described.

We claim:

1. A method of operating an electrodeposition process wherein an electrically conductive article is electrocoated from an electrodeposition bath comprising synthetic resin ionically dispersed in an aqueous medium which comprises subjecting at least a portion of the electrodeposition bath to an ultrafiltration process wherein the ultrafiltration membrane retains said dispersed resin and passes water and solute of substantially lower molecular size than said resin, returning retentate from the ultrafiltration process to the electrodeposition bath and contacting the resultant ultrafiltrate with both an ionic and a cationic ion exchange resin for a time sufficient to remove both anions and cations from said ultrafiltrate.

2. A method as in claim 1 wherein the ultrafiltration process is operated at a pressure gradient between about l and about 150 p.s.i. and wherein the ultrafiltration membrane has a flux rate of at least about 4.5 gallons per square foot per day.

3. A method as in claim 2 wherein the resin is an acid-solubilized synthetic polybasic resin.

4. A method as in claim 2 wherein the electrodeposition bath has been previously utilized to electrocoat articles.

5. A method as in claim 4 wherein a portion of the solubilized counter-ion is removed from the electrodeposition bath.

6. A method as in claim 2 wherein the resin is a base-solubilized polyacid resin.

7. A method as in claim 6 wherein the resin is a synthetic polycarboxylic acid resin.

8. A method as in claim 7 wherein the ultrafiltration process is operated at a pressure gradient between about 10 and about 150 p.s.i. and wherein the ultrafiltration membrane has a flux rate of at least about 4.5 gallons per square foot per day.

9. A method as in claim 8 wherein the electrodeposition bath has been previously utilized to electrocoat articles.

10. A method as in claim 8 wherein the cation exchange resin contains carboxylic acid groups as the functional site.

1 l. A method as in claim 8 wherein at least a portion of the excess base is removed from the electrodepositable composition.

12. A method as in claim 11 wherein the base is potassium hydroxide.

13. In an electrodeposition process wherein an electrically conductive article is electrocoated in an electrodeposition bath, comprising a synthetic resin ionically dispersed in an aqueous medium, and subsequently rinsed, the improvement comprising subjecting at least a portion of the electrodeposition bath to an ultrafiltration process wherein the ultrafiltration membrane retains said dispersed resin and passes water and solute of substantially lower molecular size than said resin, returning retentate from the ultrafiltration process to the electrodeposition bath, contacting the resultant ultrafiltrate with both an anionic and a cationic ion exchange resin for a time sufficient to remove both anions and cations from said ultrafiltrate, and utilizing the resultant liquid as at least a portion of the rinsing agent.

14. A method as in claim 13 wherein the electrodeposition bath has been previously utilized to electrocoat articles.

15. A method as in claim 13 wherein the rinsing is conducted in such a manner that rinsing removes dragout and the dragout-containing rinse agent returns to the electrodeposition bath.

16. A method as in claim 13 wherein a portion of the solubilizing counter-ion is removed from the electrodeposition bath.

17. A method as in claim 13 wherein the resin is an acidsolubilized synthetic polybasic resin.

18. A method as in claim 13 wherein the resin is a base-solu bilized synthetic polyacid resin.

19. A method as in claim 18 wherein the ultrafiltration process is operated at a pressure gradient between about l0 and about [50 psi. and wherein the ultrafiltration membrane has a flux rate of at least about 4.5 gallons per square foot per day.

20. A method as in claim 19 wherein the rinsing is conducted in such a manner that rinsing removes dragout and the dragout-containing rinse agent returns to the electrodeposition bath.

l t t t :l 

2. A method as in claim 1 wherein the ultrafiltration process is operated at a pressure gradient between about 10 and about 150 p.s.i. and wherein the ultrafiltration membrane has a flux rate of at least about 4.5 gallons per square foot per day.
 3. A method as in claim 2 wherein the resin is an acid-solubilized synthetic polybasic resin.
 4. A method as in claim 2 wherein the electrodeposition bath has been previously utilized to electrocoat articles.
 5. A method as in claim 4 wherein a portion of the solubilized counter-ion is removed from the electrodeposition bath.
 6. A method as in claim 2 wherein the resin is a base-solubilized polyacid resin.
 7. A method as in claim 6 wherein the resin is a synthetic polycarboxylic acid resin.
 8. A method as in claim 7 wherein the ultrafiltration process is operated at a pressure gradient between about 10 and about 150 p.s.i. and wherein the ultrafiltration membrane has a flux rate of at least about 4.5 gallons per square foot per day.
 9. A method as in claim 8 wherein the electrodeposition bath has been previously utilized to electrocoat articles.
 10. A method as in claim 8 wherein the cation exchange resin contains carboxylic acid groups as the functional site.
 11. A method as in claim 8 wherein at least a portion of the excess base is removed from the electrodepositable composition.
 12. A method as in claim 11 wherein the base is potassium hydroxide.
 13. In an electrodeposition process wherein an electrically conductive article is electrocoated in an electrodeposition bath, comprising a synthetic resin ionically dispersed in an aqueous medium, and subsequently rinsed, the improvement comprising subjecting at least a portion of the electrodeposition bath to an ultrafiltration process wherein the ultrafiltration membrane retains said dispersed resin and passes water and solute of substantially lower molecular size than said resin, returning retentate from the ultrafiltration process to the electrodeposition bath, contacting the resultant ultrafiltrate with both an anionic and a cationic ion exchange resin for a time sufficient to remove both anions and cations from said ultrafiltrate, and utilizing the resultant liquid as at least a portion of the rinsing agent.
 14. A method as in claim 13 wherein the electrodeposition bath has been previously utilized to electrocoat articles.
 15. A method as in claim 13 wherein the rinsing is conducted in such a manner that rinsinG removes dragout and the dragout-containing rinse agent returns to the electrodeposition bath.
 16. A method as in claim 13 wherein a portion of the solubilizing counter-ion is removed from the electrodeposition bath.
 17. A method as in claim 13 wherein the resin is an acid-solubilized synthetic polybasic resin.
 18. A method as in claim 13 wherein the resin is a base-solubilized synthetic polyacid resin.
 19. A method as in claim 18 wherein the ultrafiltration process is operated at a pressure gradient between about 10 and about 150 p.s.i. and wherein the ultrafiltration membrane has a flux rate of at least about 4.5 gallons per square foot per day.
 20. A method as in claim 19 wherein the rinsing is conducted in such a manner that rinsing removes dragout and the dragout-containing rinse agent returns to the electrodeposition bath. 